The present invention relates to a battery monitoring system for electric power tool that monitors a status of a battery inside a battery pack. The battery is used as a driving power source for the electric power tool. This invention also relates to a battery pack for electric power tool and a battery charger for electric power tool, both of which constitute the above system.
In a battery pack for electric power tool using a battery which includes a lithium-ion rechargeable battery (hereinafter also referred to as “a battery pack”), a monitoring circuit is generally included. The monitoring circuit is to always monitor a status of the battery. Items to be monitored by the monitoring circuit includes, for example, a voltage of each battery cell composing the battery, a decreasing degree of a voltage of the battery, a temperature of each battery cell for a temperature of the entire battery), a charge/discharge current to/from the battery, and the like.
It is generally configured that the monitoring circuit monitors the battery while the battery is charged and while the battery is discharged to the electric power tool which is an object to be supplied with electric power from the battery. In view of the above, the battery is used as a power source of the monitoring circuit.
In the battery pack including the monitoring circuit that is operated by using the battery as the power source as described above, it is inevitable that electric power of the battery is always consumed by the monitoring circuit although in a small amount. Consequently, even if the battery is not supplying electric power to the power tool and is simply left unused, discharging of the battery continues. As a result, a remaining battery capacity gradually decreases and eventually becomes empty. When the battery capacity decreases, the voltage of the battery also decreases. Furthermore, the monitoring circuit may become unoperational.
A method of inhibiting a decrease in the battery capacity is, for example, as follows. One method is, when the battery pack is not in use, e.g., when the battery pack is simply left unused, the monitoring circuit is switched to a sleep mode so as to stop part or all of operations of the monitoring circuit. Another method is, when the voltage of the battery falls below a certain level, supply of the electric power to each circuit inside the battery pack from the battery is completely stopped. No matter what method as such is used, however, due to natural discharge (self discharge) of a battery, even in a case that the battery is simply left unused, the voltage of the battery will be gradually decreased in the long term. Because of this, the battery is brought into an overdischarge state. As a result, the monitoring circuit becomes unoperational due to an insufficient voltage of the power source.
A battery charger that charges the battery pack including the monitoring circuit is generally configured to control charging based on a monitoring result (i.e., a status of the battery) of the monitoring circuit. However, in the battery charger configured as above, if charging is performed to the battery pack whose monitoring circuit is unoperational due to a decrease in the voltage of the battery, the following problem may occur. That is, since it is impossible to obtain the monitoring result from the battery pack, charging may not be performed normally or not be performed at all.
In this regard, as a method for charging a battery pack in which a voltage of a battery is decreased such that a monitoring circuit becomes unoperational, Unexamined Japanese Patent Publication No. 2007-82379 discloses the following method: if the voltage of the battery is equal to or less than a predetermined value, charging is started regardless of a monitoring result by the monitoring circuit, an attainment condition of the monitoring result, etc. In this method, after the voltage of the battery is recovered, a monitoring operation is started.
However, when charging is performed to a battery pack which is discharged such that the monitoring circuit becomes unoperational, for example, due to the battery pack being left unused for a long period of time, if charging is performed regardless of operations of the monitoring circuit in the same manner as the method described in Patent Publication, the following problem may arise. That is, when some abnormality of the battery occurs, or even when some abnormality of the battery had already occurred before starting the charging, the charging may be continued without the abnormality being detected.
There are various abnormalities which may occur in a battery in an overdischarge state. One example of the abnormalities is an internal short-circuit developed in a battery cell which constitutes a battery. When charging is performed to the battery having the battery cell which has developed the internal short-circuit as above, a voltage of the battery as a whole may not be recovered to a normal value. Moreover, since charging is continued, some normal battery cells may be further continued to be charged and be brought into an overcharge state.
Thus, it is not desirable to continue charging regardless of an operation of a monitoring circuit. In other words, it is not desirable to continue charging, despite the possibility that abnormality may have occurred in a battery cell constituting a battery.
The above problem may occur in not only a battery pack having a battery cell which is composed of a lithium ion rechargeable battery, but also various battery packs having a monitoring circuit which monitors a status of a battery.
In one aspect of the present invention, it is preferable that, in a battery pack including a monitoring circuit operated by a battery as a power source, if a voltage of the battery is decreased such that the monitoring circuit becomes unoperational, the monitoring circuit is operational at least when the battery is charged.